(1) Field of the Invention
The present invention relates to an optical range tracking array for receiving acoustic tracking signals and to a system for tracking vehicles using said optical range tracking array.
(2) Description of Prior Art
Optical acoustic sensors are known in the art and have been used in a number of different environments for a number of different purposes. U.S. Pat. Nos. 4,313,185 to Chovan; 4,320,475 to Leclerc et al.; 4,422,167 to Shajenko; 4,751,690 to Krueger; 4,799,202 to Assard; 4,882,716 to Lefevre et al.; 4,893,930 to Garrett et al.; 5,155,548 to Danver et al.; 5,155,707 to Fisher; and 4,688,200 to Poorman et al. illustrate various types of optical acoustic sensors known in the art and their uses.
The Chovan patent relates to an acoustic vibration sensor employing a pair of single mode optical fibers, optically coupled by a path whose length is varied by the acoustic vibrations. The sensor further includes a partially reflecting discontinuity at the sensitive end of each fiber. Optical signals of one frequency are supplied to one fiber, and of another frequency to the other fiber. Optical signals of the same difference frequency emerge from the dry end of each fiber. When these two emergent signals are photodetected, and the phase or frequency difference is obtained, the acoustic vibration is sensed.
The Leclerc et al. patent relates to a monomodal optical fiber hydrophone operating by the elastooptical effect. The hydrophone has an interferometer structure incorporating a measuring arm in which is provided a very long monomodal optical fiber immersed in the interaction medium in which is propagated the acoustic wave. A phase displacement is induced on the optical wave by the elastooptical effect and said wave is propagated in the fiber by the acoustic wave which creates an acoustic pressure field in the medium. A reference arm establishes a reference optical path and the phase displacement: linked with the acoustic wave is detected by interferometry between the two optical waves emerging from the two arms.
The Shajenko patent relates to a wide area acousto-optic hydrophone which uses signal and reference laser beams together with interferometric methods for detecting underwater acoustic signals. The signal beam is distributed across the wide sensing area of the hydrophone using beam folding techniques while being directly transmitted through a sensing chamber filled with an optically transparent bulk material, the refractive index of which varies with the incident acoustic pressure thereby modulating the signal beam. Concurrently, a reference beam of equal length and folded in an identical pattern is directly passed through an adjacent chamber filled with the same bulk material. A microhole joins the two chambers to expose the reference beam to the same static pressure and temperature fluctuations as the signal beam, thus serving as a low pass filter. The modulated signal beam and the unmodulated reference beam are then combined and superimposed on the surface of a photodetector, the output of which is proportional to the phase shifts produced by the incident acoustic signal.
The Krueger patent relates to a fiber optic interferometric hydrophone based on the change in optic path length of optic fibers bonded to both sides of a bending beam which acts as an acoustically sensitive diaphragm. The bending beam is segmented into an even number of segments, acoustic windows are arranged such that opposite sides of the beam see the acoustic field in adjacent segments, and the optic fibers cross from one side of the beam to the other to maintain the phase of strain signal for the two fibers, one on each side, along the full length of the bending beam.
The Assard patent relates to a cylindrical interferometric hydrophone having an axial hollow free-flooded volume which includes an outer fiber wrap for a sensor leg and an inner fiber wrap for a reference leg. Both inner and outer fiber wraps are wound on elastomers and exposed to a fluid medium for sensing acoustic signals.
The Lefevre et al patent relates to an optic fiber hydrophone which comprises a wide spectrum optical source, a super-luminescent diode associated with an optical fiber assembly comprising chiefly a side-hole optic fiber, subjected to the field of pressure, forming a sensor, transmitting the optic radiation in the slow and fast modes respectively, and a second fiber, not subjected to the field of pressure, the neutral axes of which are oriented with respect to the neutral axis of the first fiber so that the wave transmitted in slow mode in one fiber is transmitted in fast mode in the other fiber. The two fibers may be connected by a polarization maintaining fiber, which enables the creation of interferences, in the event that the sensor is offset with respect to the source.
The Garrett et al. patent relates to a multiple axis, fiber optic interferometric seismic sensor. The mechanical vibration transducer includes a seismic mass supported by a plurality of cylindrical silicone rubber mandrels. Each mandrel is wound with a length of optical fiber which has a reflective end and a transmissive end. A case surrounds the assemblage and is connected to the supports. When the case is displaced, the supports change diameter in response to the relative motion between the seismic mass and the case. This change in diameter is translated to a change in length of the optical fiber. By using the fibers as arms of a Michelson interferometer, a sensitive instrument responsive to displacing vibrations is obtained. This instrument is energized entirely by light transmitted through optical fiber waveguides and whose information is transmitted to the observer using only light waves in optical fibers.
The Danver et al. patent relates to a mismatched path length fiber optic interferometer which is optically coupled to an optical fiber and configured to form an omnidirectional acoustic sensor. A second mismatched path length fiber optic interferometer is optically coupled to the optical fiber and configured as a first gradient sensor. A second fiber optic gradient sensor is also optically coupled to the optical fiber. A detector optically coupled to the omnidirectional acoustic sensor and to the gradient sensors converts optical signals output therefrom to electrical signals indicative of the magnitude and direction of changes in an acoustic field. The omnidirectional acoustic sensor may include a length of optical fiber wrapped around the housing while the gradient sensors are mounted inside the housing. The housing has a volume that is adjustable for controlling the buoyancy thereof. Each gradient sensor preferably comprises a pair of mandrels formed to enclose chambers. Optical fiber coils are formed on the mandrels. Both the chambers are filled with a fluid and placed in fluid communication through a tube that defines a sensing axis between the mandrels such that acceleration of the housing along the sensing axis causes a fluid pressure differential on the first and second optical fiber coils.
The Fisher patent relates to an omnidirectional hydrophone having a pair of fiber optic windings wrapped around a resilient ball to form a spherical acoustic sensor. The fiber-optic pair has a first fiber which has a bonded jacket and a second fiber which has an unbonded jacket. The fiber with the bonded jacket is sensitive to both vibrations of the mounting structure and impinging acoustic signals. The fiber with the unbonded jacket is sensitive to vibrations but insulated from impinging acoustic signals. The hydrophone detects acoustic signals by detecting the phase difference between the two fibers.
The Poorman et al. patent relates to an optical system for detecting acoustic wave energy in a fluid medium in which coherent radiation from a laser is coupled to unequal length optical paths exposed to modulation by the acoustic energy weave generated by the sound source. The reflected beams from the paths are crosscoupled to generate interference fringes in two output beams out of phase with each other. The fringes in one output beam are counted in an up/down counter to determine the magnitude of the pressure as a function of time. The direction of the pressure change is determined by examination of the phase relationship between the fringes in the output beams. Peaks and valleys in the pressure are detected as phase reversals between the fringes in the output beams by detecting the beginning and end of a fringe in one beam without detecting the beginning or end of a fringe in the other beam therebetween. The direction of counting of the counter is reversed upon detection of a peak or valley in the modulating pressure to maintain the count as an accurate representation of the magnitude of the pressure.
U.S. Pat. No. 4,115,753 to Shajenko discloses a fiber-optic acoustic array using optic hydrophones in which sound waves are sensed and displayed as modulated light signals. The light signals so generated are transmitted along the fiber-optic bundles.
U.S. Pat. No. 4,311,391 to Gilmour illustrates a passive. fiber optic sonar system wherein first and second optical fibers are wound on a common mandrel and provided with a light energy beams. An acoustic signal differentially varies the index of refraction of the optical fibers to result in an interference pattern dependent upon the frequency of the received acoustic signal or signals.
U.S. Pat. No. 4,649,529 to Avicola relates to a multi-channel fiber optic sensor system including two or more sensors formed on an optical fiber and a phase sensitive detector. Each sensor includes two reflectors separated by a section of the fiber. Each reflector may be activated so that when an interrogating light signal propagates in a first direction past the activated reflector, a portion of the interrogating light signal will be reflected back into a direction opposite the first direction. Each reflector may also be deactivated so that the interrogating signal may propagate unhindered past the deactivated reflector. Variations in the optical path length between the reflectors of a sensor, due to changes in an external parameter of interest, will cause phase modulations that are extracted in the phase sensitive detector by homodyne or heterodyne techniques.
U.S. Pat. No. 5,051,965 to Poorman relates to an acousto-optical seismic sensor array including a distributed set of optical fiber sensing coils. A light pulse is launched through the sensing coils in serial order. The light pulse is cumulatively data-modulated by the respective sensing coils and is returned as a time division multiplexed pulse train. The pulse train is split into a first pulse train and a retarded second pulse train. The retardation time equals the travel-time delay of a light pulse between sensors. The retarded pulse train is compared with the first pulse train to determine the phase shift therebetween for consecutive pulses. The phase shift is an analog of the quantity being sensed.
In past designs, portable shallow water tracking sensor arrays have consisted of electrical, mechanical and/or optical components. The electronics perform the complex function of interfacing to an acoustic sensor and transmitting information to the processing system. The fact that electronics were required, complicated the design in the following technical areas. By forcing water tight integrity of the electronic components, the mechanical housing had to be designed and fabricated such that no sea water intrusion occurs at the hydrostatic pressure where the system will be used. Further, the housing must remain dry at all times.
With the advent of optical long haul communications technology into undersea range tracking systems, the in-water design is further complicated. The hydrophone sensor node interconnect and trunk cable designs are also complex because of the joining and terminating of the electrical, mechanical and optical properties required in the cable. Termination of the cable is a perplexing task involving several intricate steps in order to keep the sea-water from intruding into the system. All these processes increase the overall system cost, the labor and skill required to perform the assembly processes due to the complex designs.
There are times when portable shallow water tracking sensor arrays are to be installed in harsh ocean environments due to testing requirements. The harsh shallow water ocean environments include areas of rough bottom types such as coral and where local bottom fishing operations occur. In such environments loss of part of the system is a real possibility during the course of the installation period. It would be a big advantage to develop a system that was as inexpensive as possible so that the financial loss of equipment would be minimized.